1. Field of the Invention
The present invention relates to a hydraulic apparatus for a heavy construction equipment, and more particularly to a variable priority device for a swing motor in a heavy construction equipment.
2. Description of the Prior Art
Where at least two actuators are operated by a single pump, a "priority" is established to control the actuators such that one of the actuators is supplied with a larger amount of oil than the other actuator. For example, in an excavator wherein a swing actuator, namely, a swing motor, and an arm actuator, namely, an arm cylinder are supplied with operating oil from a single pump, the amount of oil required for swing operation is considerably larger than the amount of oil required for an arm operation. To this end, a priority is assigned to the swing motor by limiting the amount of oil supplied to the arm cylinder and increasing the amount of oil supplied to the swing motor by the limited amount of off. Such a relationship is also applied to a case wherein a boom should have priority over a bucket.
Meanwhile, a stroke limiter or an orifice is conventionally used to provide the above-mentioned priority function. In the case illustrated in FIG. 1, wherein the stroke limiter 101 is used, the stroke limiter 101 is equipped in an arm cylinder control valve 103 to limit a spool stroke of the arm cylinder control valve 103 within a certain range, thereby preventing a supply oil passage 105 defined in the valve 103 from being fully opened. In the case illustrated in FIG. 2, wherein the orifice 201 is used, the orifice 201 is disposed in an arm cylinder-side parallel oil passage 203 to limit the amount of oil supplied to an arm cylinder control valve 205.
In either case, namely, where the supply oil passage defined the arm cylinder control valve is prevented from being fully opened by the stroke limiter, or where the arm cylinder-side parallel oil passage is limited on the oil amount passing therethrough by the orifice, the amount of oil supplied to the arm cylinder is reduced, while the limited amount of oil is additionally supplied to the other actuator, namely, the swing motor. As a result, the swing motor has priority over the arm cylinder.
However, the above-mentioned conventional devices have a problem in that the oil passage is always limited to the amount of oil passing therethrough when there is only one actuator in operation, or a combined operation of both actuators. In other words, although the swing motor desirably has priority over the arm cylinder by the function of the stroke limiter or orifice when both the swing motor and the arm cylinder operate in a combined manner, the arm cylinder-side oil passage is also limited undesirably on the amount of oil passing therethrough even when the arm cylinder operates alone. In the latter case, limiting the oil amount passing the arm cylinder-side oil passage results in various problems such an unnecessary loss of pressure, heat generation caused by overload and a decrease in the operating speed of the arm.